The present invention relates generally to peristaltic pumps and more specifically to peristaltic pumps used in ophthalmic surgical equipment.
Most prior art peristaltic pumps work by compressing or squeezing a length of flexible tubing (sometimes between a fixed race) using a rotating roller head. As the roller head rotates, the rollers pinch off a portion of the tubing and push any fluid trapped in the tubing between the rollers in the direction of rotation. Peristaltic pumps are widely used in medical applications because of their predictable, constant flow properties. These prior art systems, however, typically require manual connection of the pump tube segment around the rotating roller head.
Prior art peristaltic pumps using rotating roller heads also typically impart unwanted pressure pulsations. Several pulsation damping devices have been developed to address this problem (see e.g., U.S. Pat. No. 4,921,477 (Davis)).
With respect to cassettes used in surgical systems having a venturi-type aspiration pump, for the cassette to act as an effective reservoir, the level of fluid (and thus the empty volume) within the cassette must be controlled so that the cassette is neither completely filled nor emptied. If fluid fills the cassette in an aspiration system, fluid may be drawn into the venturi, which would unacceptably interfere with the vacuum level at the surgical instrument. An empty cassette in an aspiration system will result in air being pumped into the drain bag, which would waste valuable reservoir space within the bag. Moreover, constant volume within the cassette in an aspiration system enables more precise control of the level of vacuum within the surgical instrument.
Additionally, the size of the reservoir within the cassette affects the response time of the cassette. A larger reservoir provides more storage capacity but slows the response time of the system. A smaller reservoir increases the response time of the system, but may not have adequate storage capacity. This dilemma has been addressed by cassettes that have two internal reservoirs. Such a cassette is illustrated in U.S. Pat. No. 4,758,238 (Sundblom, et al.) (the “Sundblom Cassette”). The smaller reservoir is in direct fluid communication with the surgical handpiece while a larger reservoir is positioned between the smaller reservoir and the source of vacuum. This allows for a faster response time and larger storage capacity. The smaller reservoir, however, must be periodically emptied into the larger reservoir prior to the smaller reservoir filling up. This requires that the vacuum to the smaller reservoir be closed prior to opening the drain to the larger reservoir. Closing the vacuum line to the small reservoir requires that the surgical procedure be stopped during drainage of the smaller reservoir.
Another system, described in U.S. Pat. No. 5,899,674 (Jung, et al.) uses a venturi pump to provide aspiration vacuum and a separate peristaltic pump located between the large reservoir and the small reservoir. The peristaltic pump is used to actively pump the fluid from the small reservoir into the small reservoir when the small reservoir becomes full. Such a system allows for the continued operation of the system during drainage of the small reservoir.
Another prior art pump is disclosed in U.S. Pat. No. 6,293,926 B1 (Sorensen, et al.) that describes a peristaltic pump having a molded flow channel contained on an elastomeric sheet that is bonded or mechanically attached to a rigid substrate. The pump head rollers are mounted radially from the axis of rotation of the pump motor and compress the elastomeric flow channels against the rigid substrate. The commercial embodiment of the invention described in this patent is sold as the INFINITI® Vision System by Alcon Laboratories, Inc., Fort Worth, Tex. This surgical console uses a Fluid Management System or cassette wherein the elastomeric sheet is friction fit into the rigid substrate without the use of any adhesives Such a construction method has proven to be extremely reliable, but this commercial system is intended primarily for cataract surgery and has no other source of vacuum other than the peristaltic pump. Therefore, the fluidic system is exposed primarily to negative pressure or vacuum and is not exposed to transient high positive pressures, such as are encountered during a reflux operation in posterior segment surgical procedures using a venturi pump as the primary source of vacuum. High positive pressures have the potential to cause failure at the elastomer/substrate interface if a friction fit construction technique is used.
Accordingly, a need continues to exist for a method of operating a peristaltic pump that reduces the potential for cassette failure under transient positive pressures.